The objective of this proposal is to study the uptake, degradation and plasma clearance of antagonists of luteinizing hormone- releasing hormone (LHRH) by relevant rat organs in vivo, and to evaluate the effects of altering antagonist molecular charge on these processes. Antagonists will be designed to define which molecular sites with one or more cationic charges enhance antagonist potency. The half-life for plasma clearance of antagonists will be determined in the systemic circulation. Renal handling of a labeled antagonist will be studied by its microperfusion into isolated nephron segments in vitro and microinfusion into renal surface tubules in vivo, and by its arterial infusion into filtering and nonfiltering kidneys in vivo. Labeled products will be identified and quantified by high performance liquid chromatography of bathing medium, collection fluid, urine, and venous blood where pertinent. These studies will elucidate the effect of an enzymatically resistant LHRH analog and of antagonists with altered molecular charge, on uptake, degradation, and transport by tubular and peritubular processes. Cellular sites for hydrolysis and transport of antagonists will be determined with subcellular fractions and by electron microscopy-autoradiography. The antagonists will be administered by gavage, and the resistance to breakdown by GI peptidases and absorption of intact peptide and/or metabolites into the systemic circulation and/or lymphatics will be evaluated. The antagonist will be perfused through the brain and liver circulations in vivo to study mechanisms for degradation of peptides by the vasculature. Sequestration and degradation of antagonists in the liver and the excretion of antagonist and/or metabolites in bile will be investigated. The potency of an antagonist will be determined by: a. the screening program of the Center for Population Research, Contraceptive Development Branch, National Institutes of Health; b. an in vivo assay which discriminates its capability to selectively inhibit LH or FSH release; c. binding studies with pituitary cell membranes. These studies would: a. advance our understanding of mechanisms of LHRH antagonists transport, hydrolysis and excretion in pertinent organs; b. quantify the role of various body organs in controlling the plasma levels of antagonists; c. permit a more rational approach to the design of enzymatically resistant antagonists with prolonged activity.